Method and apparatus for forming dye sublimation images in solid plastic

ABSTRACT

Method for forming a dye sublimation image in a substrate, particularly a plastic substrate, and apparatus to perform the method. According to the method taught by the present invention, a substrate is disposed on a perforated platen and a dye carrier having an image formed thereon of a dye sublimation ink is disposed on the substrate such that the image is in contact with the substrate. The substrate, dye carrier and at least a portion of the perforated platen are then covered with a flexible membrane. A clamping pressure is then applied to the substrate and the dye carrier through the membrane. Once the clamping pressure is attained, the substrate and dye carrier are first heated, then cooled. 
     The invention disclosed herein further teaches a number of novel apparatus to effect the method, resulting in significantly superior dye sublimation imaging than heretofore attainable in rigid plastic sheets.

FIELD OF THE INVENTION

The present invention relates to the formation of images within solid sheets of plastic. More particularly, the present invention relates to a methodology for forming dye sublimation, or dye transfer, images within solid sheets of plastic and to an apparatus for performing the methodology.

BACKGROUND OF THE INVENTION

From the advent of plastics, users and manufacturers thereof have sought a workable means for imprinting or forming images thereon. Prior imaging technologies suitable for use on other materials, for instance metals, wood, and the like, have not generally met with success when used to perform permanent imaging plastics. Examples of such prior imaging technologies include but are not limited to paints, decals, lacquers, and dyes. In general the problems associated with utilizing prior imaging or marking technologies center on certain chemical and physical properties of plastics in general.

One of the great advantages of plastics is that they can be formed into complex shapes having inherently very smooth surfaces. While this is an advantage in the manufacture of such plastic objects, the extremely smooth and often chemically resistant nature of plastic surfaces renders the application thereto of paints and the like less than satisfactory. Many paints, for instance enamels, when applied to plastics tend to flake or peel when the plastic is flexed or when the image is subjected to physical distress such as abrasion or temperature change.

In searching for a methodology for forming permanent, abrasion-resistant images in sheet plastics workers in this field have noted that plastics tend to be molecularly similar to certain fabrics which are imaged utilizing a dyeing process known as “dye sublimation”. According to known dye sublimation processes, an image, for instance a decorative design, is formed of sublimation printing inks on a dye carrier, sometimes also referred to as a transfer paper or auxiliary carrier. Dye carriers are often, but not exclusively, formed of paper. Printing the image on the dye carrier is carried out by any of several known printing methods including, but specifically not limited to offset or rotary printing methods. The print images formed on the dye carrier are transferred by sublimation, also called transfer printing, from the dye carrier to the textile or fabric which is to be decorated with the design.

There are several known dyestuffs suitable for use with dye sublimation printing techniques. The actual dyestuff or dye carrier utilized is not essential to the principles of the present invention, provided that the dyestuff is capable of sublimation. This is to say that the dyestuff sublimates directly to the vapor state from the solid state upon the application of heat. One type of printing ink suitable for sublimation printing is prepared from sublimable dyestuffs utilizing binders and oxidation additives. The term “sublimable” is defined herein to mean capable of sublimation.

Currently, to form a dye sublimation image in a textile, the printed dye carrier is placed with its color-imprinted side on the textile face to be imprinted and is thereafter heated. As soon as the dyestuffs reach a temperature of from about 170 to about 220 degrees Celsius, those dyestuffs sublime into the textile and the desired image is thereby formed in that textile.

From the foregoing discussion, it will be appreciated that one of the advantages of dye sublimation printing is that the image is actually formed within the structure of the textile, or substrate, on which it is imprinted. This is in direct contrast to most printing techniques, wherein the image is formed solely on the Surface of the substrate. While surface-formed images are completely suitable for many applications, they are less than optimal for others. By way of illustration, in the preceding discussion of dye sublimation images formed in textiles, it will be appreciated that if a textile is subjected to substantial wear, as is a carpet, an image formed solely on the surface of that carpet, or on the surface of the individual carpet fibers, will tend to wear quickly.

It will further be appreciated that most inks suitable for forming surface images tend to be opaque. Again, this is perfectly suitable for many applications. However, where it is desirous that the resultant article has a lustrous or translucent property, the use of such opaque inks precludes the desired translucent image.

U.S. Pat. No. 3,649,332 to Dybvig discloses an early attempt at transfer printing of plastics. According to '332, a photo-sensitive dye carrier having an image formed thereon is placed against a porous paper temporary receptor sheet on a vacuum platen and sufficient vacuum is established to hold the two sheets in close contact and in fixed position. The transfer sheet has a dye coating on the surface contacting the receptor sheet and a photoconductive zinc oxide coating on the outer surface. The outer surface is exposed to a color separation light image from a positive color original, to impart a latent image.

A conductive roller carrying a coating of conductive radiation-absorptive toner particles at a high potential is passed over the exposed surface to deposit toner at the non-light-struck areas. The surface is then briefly exposed to intense infrared radiation causing transfer of dye to the receptor at the infrared absorptive toned areas. The vacuum is then released, and the photosensitive sheet is removed and replaced with a second photo sensitive sheet carrying a second dye, and the process is repeated utilizing an appropriate color separation filter. This process is again repeated using a third filter and sensitive sheet to produce a full three-color intermediate.

One or more portions of the intermediate are then cut from the sheet. These segments are placed against a transparent dye-receptive film in a desired arrangement, and over them is placed a paper dye source sheet having a blue dye coating as previously described, but minus the photoconductive coating of the transfer sheet. The three layers are pressed together and briefly heated. Thereafter the film is removed and is found to retain a brilliantly clear, full-color copy of the detail sections on an equally clear blue background.

U.S. Pat. Nos. 4,059,471, 4,202,663, and 4,465,728 to Haigh, or Haigh et al. detail methodologies for forming dye transfer images in plastic surfaces, especially thin films. These several patents flow either directly from, or as a divisional or continuation in part of, U.S. patent application Ser. No. 540,383 filed Jan. 13, 1975. Each of these patents utilize a dye transfer process for forming a dye pattern on a dye receptor plastic web, most especially thin films of from 2 to 20 mils in thickness, by interposing a carrier web, for instance a polyolefin carrier web, between the dye receptor plastic web and a transfer web containing dispersed dyes. Thereafter the several webs are pressed together in close contact and are heated to a sublimation temperature suitable for the dyes, and the several webs are maintained at the sublimation temperature until a substantial portion of the dyes have sublimed and transferred from the transfer web through the polyolefin web to the dye receptor web. Thereafter the several webs are cooled below the softening temperature of the dye receptor web, and the dye receptor web is separated from the other webs.

U.S. Pat. No. 4,242,092 to Glover teaches a method of sublimatic printing on air-permeable sheet structures such as carpets or tiles. According to '092, an air-permeable sheet structure is imprinted by placing an air-permeable printing foil carrying on one side thereof a sublimatic dyestuff in a face-to-face relationship, and in close proximity, with the air-permeable sheet structure. The side of the foil having the dyestuff imprinted thereon is placed in contact with the air-permeable sheet structure, and the foil is heated at a temperature and for a period of time suitable to vaporize the dyestuff. At the same time a gas or vapor pressure differential is applied so as to create a flow of air from a space above the foil, and through both the foil and the sheet structure, thereby causing the dyestuff vapor to flow into the sheet structure and to form an image therein.

U.S. Pat. No. 4,662,966 to Sumi et al. teaches an apparatus for transfer printing a plurality of articles, for instance typewriter keys, which are held on a plane in rows and then heated. '966 discloses that this apparatus further includes conveyors for conveying the plurality of articles to a heating outlet, the heating outlet having infrared radiation heaters provided inside. The apparatus further includes a holding device for holding the articles at a predetermined position with respect to the article holder. Another holder is designed to hold a transfer sheet at a second predetermined position. The transfer sheet has a pattern layer formed thereon of thermo-diffusable dye. There is also provided a means for pressing the transfer sheet against the articles so that the pattern is transfer-printed on the articles, and a conveyor for conveying the article holder with the plurality of articles thereon through the heating apparatus and the various holding devices.

U.S. Pat. No. 4,664,672 to Krajec et al. teaches a method for transfer printing onto objects made of plastic, or having a plastic surface coating, by pressing a thin dye carrier on the surface to be printed during the dye transfer process. This is effected by means of super-atmospheric gas pressure, whereby the surface is kept at a temperature below the thermoplastic range of the plastic object. According to the methodology taught by '672 a dye carrier, for instance a paper dye carrier, is pre-dried below the sublimation temperature of the ink. The dye carrier is clamped, for instance in a spectacle frame in close proximity above but not touching the surface to be printed. Thereafter a gas under pressure is applied to the backside of the carrier, which gas exerts a slight super-atmospheric pressure directly or indirectly against the backside of the dye carrier, pressing the carrier against the object. Thereafter a heat source, for instance a heat radiator, is placed so that its radiation is directed toward the backside of the dye carrier.

U.S. Pat. No. 5,308,426 to Claveau teaches a process for forming sublimation images on objects, evidently irregular non-planar objects, by forming an “ink support” from a material which is both extensible and air permeable and which will conform to the shape of the object. This ink support is used to envelop the object, which is then placed in a vacuum machine. The vacuum machine, with the ink support inside, is then introduced into a heated space, causing transfer of the decoration over the whole surface of the object be decorated. Examples of extensible air-permeable materials suitable as ink carriers for utilization in the '426 invention include woven fabrics, knitted fabrics, and sheets of non-woven material.

U.S. Pat. No. 5,997,677 to Zaher teaches a methodology for applying a colored decorative designed on a plastic substrate by heating the carrier and then placing the carrier in contact with the substrate by air suction, such that a sub-pressure results between the carrier and the substrate. Thereafter an inhomogeneous exposure of infrared radiation is directed to the carrier in correspondence with the prevalent color portion of the dyestuff to which the radiation is applied. The dye carriers taught by '677 “ . . . above all are sheets of paper which, on the one hand, are good at accepting the images of sublimable dyes to be transferred and, on the other hand, are sufficiently permeable so that air can be sucked through the dye carrier . . . during sublimation transfer printing.”

Many of the known dye sublimation printing methodologies applied to solid plastics are so sensitive to variations in pressure, temperature, dye lot, substrate lot, and other manufacturing variables, that at least one inventor has directed his inventive efforts solely to the task of pre-conditioning a plastic substrate for dye sublimation printing. This pre-conditioning is taught and explained in U.S. Pat. No. 5,580,410 to Johnston.

Given that the formation of precise, vibrant, durable images in solid plastic sheets is a long-sought goal of the plastics imaging industry, why are there currently no flat solid sheets of plastic which have been imprinted utilizing this methodology, which sheets are formable into commercial articles? The lack of success on the part of other inventors in this field is largely due to the fact that while the inventions disclosed in the previously discussed patents may theoretically be capable of implementation, in actual practice their use has failed to produce imaged flat plastic sheets at commercially acceptable costs or in commercially acceptable volumes. There are several reasons for this lack of success.

The first reason that many known processes have not resulted in commercially successful imaged articles is that they are slow. An imaging process which requires an extended period of time to successfully form an image, or which requires a large number of complex and delicate steps to effect, may result in a successfully imaged flat plastic sheet, but one whose imaging is so expensive as to render it commercially non-viable. Moreover, previous imaging processes are so sensitive to temperature variations that very slight changes in processing temperatures result in unacceptable images or destroyed substrates.

The second reason that many of these known processes have failed to yield the desired result is closely related to some of these process variables previously discussed. One particularly aggravating shortcoming of many prior dye sublimation imaging processes is that, in order to form the dye sublimation image in a solid plastic substrate, that substrate must have its temperature elevated above its thermoplastic limit. In many cases this results in substantial liquefaction of the substrate, with attendant unwanted adhesion of the dye carrier to the now liquefied and sticky substrate. This of course results in a substrate having at least a portion of the dye carrier adhered thereto, often permanently. Even where it is possible to scrape the adhered dye carrier from the cooled substrate, this scraping not only results in a poor surface finish, but also requires significant cost in terms of additional man-hours to effect.

Some of the previously discussed inventions, in order to obviate the unwanted adhesion of dye carriers to sticky substrates, have relied upon placing some material between the substrate and the dye carrier. Examples of these materials include parting compounds, such as talcum, or permeable webs. The introduction of such parting or separating materials may preclude, in some instances, the unwanted adhesion of the dye carrier to the substrate, but this is done with significant degradation of the imaged article. These methodologies are admitted to cause degradation in surface finish, image resolution, or image registration on the substrate.

Finally, and most importantly, when applied to solid plastic sheets, known dye sublimation imaging processes tend to shrink, warp and distort those sheets. While the degree of shrinkage, warping, and distortion varies from process to process and substrate to substrate, these defects encountered utilizing known dye sublimation imaging technologies result in anything from mildly rumpled surfaces to wildly distorted sheets having all the planarity of potato chips. Since the object of dye sublimation imaging of solid plastic sheets is to form an image within the sheet while retaining its substantially planar nature in an un-shrunken, un-warped and distortion-free state, none of the known processes can be said to be fully successful. Moreover, one or more of the technical performance specifications of plastic sheets imaged by other dye sublimation processes are often lost be subjecting the sheets to the process. These technical performance specifications include, but are not limited to shrinkage, impact resistance, dimensionality, and mechanical strength.

What is clearly needed is a methodology for forming a durable, clear, sharp, image in a solid, flat sheet of plastic by means of a dye sublimation process that results in an un-shrunken, un-warped, distortion-free plastic sheet which retains all of the original plastic sheet's technical performance specifications.

What is further needed is a dye sublimation imaging methodology that completely obviates the unwanted adhesion of the dye carrier to the substrate during imaging.

Finally what is needed is at least one apparatus capable of performing such a methodology.

SUMMARY OF THE INVENTION

The present invention provides a methodology for forming a dye sublimation image in a solid sheet of plastic. The methodology completely obviates the unwanted adhesion of dye carriers to substrates. The imaging methodology taught herein routinely produces a durable, clear, sharp image in a flat plastic sheet, which sheet is un-shrunken, un-warped, and distortion-free and which retains all of its original technical performance specifications.

In order to attain these advantages, the present invention teaches a method wherein a substrate is disposed on a perforated platen and a dye carrier having an image formed thereon of a dye sublimation ink is disposed on the substrate such that the image is in contact with the substrate. The substrate, dye carrier, and at least a portion of the perforated platen are then covered with a flexible membrane. A uniform clamping pressure is then applied to the substrate and the dye carrier. One means of attaining this uniform clamping pressure is by inducing a pressure differential between the ambient atmosphere and the atmosphere under the membrane. Once the clamping pressure is attained, the substrate and dye carrier are first heated to a temperature sufficient to effect the dye sublimation imaging and retained at that temperature for a period of time sufficient to cause the imaging, then they are cooled to a point where the substrate again becomes rigid. The heating and cooling taught herein effect a first and a second thermal event, both performed while the substrate is under clamping pressure. The first and second thermal events, in operative combination, not only form the image and preclude the unwanted warping, twisting, or shrinkage of the substrate, but also preclude the unwanted adhesion of the dye carrier to the substrate.

The invention disclosed herein further teaches a number of novel apparatuses to effect the method, resulting in significantly superior dye sublimation imaging than heretofore attainable.

These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the Drawing.

BRIEF DESCRIPTION OF THE DRAWING

For more complete understanding of the present invention, reference is made to the accompanying Drawing in the following Detailed Description of the Invention. In the drawing:

FIGS. 1A-H are cross-sectional views through a platen assembly according to the present invention, demonstrating the method thereof.

FIGS. 2A-C are frontal views of a first apparatus for performing the method of the present invention.

FIG. 3 is a perspective view of the first apparatus for performing the method of the present invention.

FIGS. 4A-C are frontal views of a second apparatus for performing the present invention.

FIGS. 5A-B are frontal views of a third apparatus for performing the present invention.

FIG. 6 is a perspective view of the second and third apparatuses for performing the method of the present invention

FIG. 7 is a cross-section through a thermal head according to one embodiment of the present invention employing an active cooling device.

FIG. 8 is a cross-section through a thermal head according to another embodiment of the present invention employing an alternative active cooling device.

FIG. 9 is a cross-section through a thermal head according to another embodiment of the present invention employing both active and passive cooling devices.

FIG. 10 is a cross-section through a thermal head according to one embodiment of the present invention employing a passive cooling device.

FIGS. 11A-F are cross-sections taken through an alternative platen assembly.

FIG. 12 is an elevation view of a platen rack having loaded therein a plurality of alternative platen assemblies.

FIG. 13 is an elevation view of a platen rack having loaded therein a plurality of alternative platen assemblies, the platen rack being received into an oven.

FIGS. 14A-H illustrate an alternate vacuum bag implementation of the present invention.

FIG. 15 is a graph of temperature over time for one dye sublimation imaging cycle, having superimposed thereon a time line indicating the several control actions required to effect the cycle.

Reference numbers refer to the same or equivalent parts of the invention throughout the several figures of the Drawing.

DETAILED DESCRIPTION OF THE INVENTION

The succeeding discussion centers on one or more preferred embodiments of the present invention, implemented by a number of components. Those having skill in the art will understand that where the embodiments enumerated herein specify certain commercially available components, these are by way of example. The principles of the present invention are capable of implementation in a wide variety of configurations and these principles specifically contemplate all such embodiments.

Having reference now to FIGS. 1A through 1H, a methodology taught by the present invention for forming dye sublimation images in substrates, particularly in solid plastic substrates, is shown. At FIG. 1A is shown a platen, 10, having superimposed thereon a passive cooling device, 12. The principles of the present invention specifically contemplate the utilization of either or both active and passive cooling devices, as will be explained later. Platen 10, in one embodiment of the present invention, is a flat aluminum plate transfixed by a plurality of vacuum orifices 14. Vacuum orifices 14 are further connected to a vacuum system, 240. Placed atop passive cooling device 12 for purposes of forming a dye sublimation image therein is substrate, 1. In order to form the dye sublimation image, dye carrier 3 having an image, 5, imprinted thereon utilizing the previously discussed dye sublimation inks, is placed atop substrate 1.

While the succeeding discussion is directed to the dye sublimation imaging of plastic sheets and the like, the principles of the present invention may advantageously be applied to the dye sublimation imaging of a wide variety of man-made and naturally occurring sheet material substrates, including but specifically not limited to metals, stone, wood, waxes, polymers, monomers, resins, textiles, fabrics, glasses, minerals, and composites thereof. The principles of the present invention specifically contemplate all such applications.

The passive cooling device 12 of this embodiment of the present invention consists of a panel having an extremely low thermal mass, for reasons which will be later explained. One embodiment of the present invention contemplates the utilization of a hex-cell aluminum-cored composite sandwich panel having glass-reinforced plastic upper and lower surfaces. One such panel suitable for implementation as passive cooling device 12 is a Fiber-Lok No. 2330 sandwich panel available from Burnham Composites, Wichita Kans. According to this embodiment of the present invention, passive cooling device 12 is of smaller surface extent than platen 10, but is at least as broad in extent as the substrates which will be processed on it. This is necessary in order that there be at least some of the plurality of vacuum orifices 14 available to form a vacuum path for membrane 16, as will be subsequently explained.

Referring now to FIG. 1B, membrane 16 is applied over the stack comprising cooling device 12, substrate 1, and dye carrier 3. Membrane 16 further overlaps at least a portion of platen 10. Membrane 16, for ease of handling, may be fitted to a spectacle frame, not shown in this figure. Membrane 16 should be capable of forming a substantially airtight seal for purposes of clamping the substrate-dye carrier stack together in close proximity. Membrane 16 should also have sufficient strength to prevent the warping of substrate 1 during the thermal events which constitute one dye sublimation cycle and which enable dye sublimation imaging and dye carrier removal, as will be later explained.

Other properties desirable of membrane 16 are that it is substantially chemically compatible not only with substrate 1 and the sublimatic dyes imprinted on dye carrier 3, but also with any byproducts out-gassed from substrate 1 or dye carrier 3 during dye sublimation imaging.

In one embodiment of the present invention, the lower surface of membrane 16 is lightly textured to provide a continuous vacuum channel across the interface between membrane 16 and dye carrier 3 without forming bubbles between the membrane and dye carrier. These bubbles would preclude even clamping of dye carrier 3 to substrate 1. This texture also serves as a vacuum release and as a bleeder to trail off the vacuum when it is no longer needed for clamping.

Moreover, in order to smoothly mold and flow over the several elements of the cooling device-substrate-dye carrier stack, as well as to platen 10, it is desirable that membrane 16 be formed of a flexible material. When used on dye carrier-substrate-cooling device stacks having significant vertical extent, for instance greater than about one inch in thickness, it further desirable that the membrane be formed of an elastomeric material to more smoothly mold and flow over these several elements. As the imaging process taught herein utilizes rapid temperature changes, as well as sustained periods of temperatures up to 600° F., is also required of the membrane that it be not only heat-resistant, but that it be capable of withstanding repeated thermal cycles between higher and lower temperatures without hardening, cracking, loss of structural integrity or loss of any of the previously discussed properties.

From the foregoing discussion it will be appreciated that a number of materials are suitable for membrane 16. Examples of such materials include, but are specifically not limited to: vulcanized rubbers, silicones, butyl rubbers, polymers, chloropolymers, fluoropolymers, and other natural or man-made elastomeric sheets. Membrane 16 is brought into substantially continuous contact with dye carrier 3, and covers substantially all of the plurality of vacuum orifices 14 not previously covered by passive cooling device 12.

Referring now to FIG. 1C, the clamping step of one embodiment of the present invention is explained. Membrane 16 having previously been positioned over the dye carrier-substrate-cooling device stack, as well as at least a portion of platen 10 including at least one and preferably a plurality of vacuum orifices 14, now exerts an atmospheric clamping pressure, as shown at 20. As used herein, the term “atmospheric clamping pressure” denotes the use of a pressure differential between the ambient atmosphere and the atmosphere beneath the membrane to effect the clamping of the substrate and dye carrier. This atmospheric clamping pressure may be effected by means of vacuum, air pressure, or a combination of the two.

In the embodiment under discussion, atmospheric clamping pressure 20 is attained by means of connecting at least one of a plurality vacuum orifices 14 to a vacuum system, 240, and thereby applying a vacuum, as shown at 18, to the underside of membrane 16. It should be noted that vacuum system 240 has been deleted from FIGS. 1A-B and D-H for purposes of illustrational clarity. Where a substantially perfect vacuum is obtainable at sea level, this theoretically results in a clamping force of approximately 14.7 psi over the entire surface of the dye carrier-substrate stack. Practically, a perfect vacuum is seldom obtainable and in any event is not generally necessary. Clamping forces equating to 14 psi resulting from less than perfect vacuum have been found to yield dye transfer images vastly superior to those obtainable by any other methodology. Depending upon the mechanical properties of the substrate, the dye transfer temperatures, and the nature of the thermal events occasioned by the application of the principles of the present invention, even lower clamping pressures may be utilized.

While the foregoing embodiment utilizes vacuum clamping, alternative embodiments utilize other means of attaining the very even clamping pressure afforded by vacuum clamping. These alternatives include, but are not necessarily limited to the use of mechanical clamping pads incorporating a pressure-leveling layer, for instance foam rubber or sacrificial rigid foam sheets, and the use of air pressure clamps, for instance bag presses.

It should also be noted that clamping pressure, including the previously discussed vacuum clamping pressure, may serve as a processing control variable. For some imaging routines in some substrates, it may be advantageous to modify the clamping pressure above or below the nominal 1 atmosphere clamping pressure discussed above. Clamping pressures lower than 1 atmosphere may be attained and maintained by utilizing a vacuum regulator. Clamping pressures greater than 1 atmosphere may by attained by augmenting the vacuum clamping pressure with a supplementary clamping force. One methodology for attaining this latter option is by means of a bag press superposed over the membrane, the inflated force of which bag press supplements the vacuum clamping attained by the membrane alone.

Referring now to FIG. 1D, a first, or heating thermal event for forming a dye sublimation image is imposed on the membrane-dye carrier-substrate stack as follows: thermal energy is applied through membrane 16 and dye carrier 3 to substrate 1. In this embodiment of the present invention it has been found advantageous not only in terms of manufacturing efficiency, but of efficacy of later removing dye carrier 3 from substrate 1, that thermal energy 22 be provided as rapidly as possible to substrate 1. Thermal energy 22 may be applied through membrane 16 in substantially any manner known to those having ordinary skill in the art that will not damage membrane 16. The previously applied atmospheric clamping force is maintained throughout this step.

Examples of applicable heat transfer methodologies include, but are specifically not limited to: electrical resistance heating, for instance by means of electrical resistance wires embedded in membrane 16, or applied either above or below membrane 16; by the application of steam to an upper surface of membrane 16; by the application to an upper surface of membrane 16 of a flow of heated gas including steam, flame or heated fluid; or by the application of radiant energy to the top of membrane 16. Examples of such radiant energy include but are not limited to infrared energy applied by means of infrared lamps, or ultraviolet radiation, and microwave radiation. Another alternative for applying thermal energy 22 is the application to an upper surface of membrane 16 of a conductive heating source, for instance a heated plate. Again, this plate may be heated by any known heating methodologies, such as those previously discussed, as well as by introducing into a hollow interior of the plate a flow of heated fluid or gas.

Plastics, for instance thermoforming plastics, have specifically different physical attributes depending upon their internal temperature. At room temperature most commercially usable thermoforming plastics are substantially rigid, for instance as rigid sheets. At the other end of the temperature spectrum, heating a thermoforming plastic substantially above its thermoplastic temperature results in the substantial liquefaction of the plastic, with attendant destruction of the structure formed by the plastic as the plastic liquefies. Intermediate between these two extremes are temperatures at which the plastic begins to soften but is not yet fully liquid. It is at these intermediate temperatures that the dye sublimation process of the present invention is conducted.

In order to simultaneously render substrate 1 mechanically and chemically suitable for the introduction of dye, as well as to provide for the sublimation of imaging dyes from the solid to the vapor state, the temperature of the substrate, and hence the dye carrier, must be elevated beyond the plastic's rigid and generally impervious state to a state where the plastic begins to soften, and where the sublimation dyestuffs vaporize. In order to retain the structural integrity of the plastic, and to maintain the technical performance specifications of the plastic, it is necessary that it not be heated to the point where it liquefies. The ideal temperature is of course application specific and depends not only upon the type and thickness of plastic sheet to be imaged but also upon the nature of the imaging dyes.

The application of thermal energy 22 to raise the internal temperature of substrate 1 and dye carrier 3 comprises the first thermal event of the present invention. The duration of the first thermal event is again application specific and is determined empirically. One of the metrics for determining the duration of the first thermal event is the desired degree of penetration of the image into the plastic.

Referring now to FIG. 1E, following the first thermal event, which actually effects the dye sublimation imaging of the substrate 1, a second, or cooling thermal event is accomplished while the substrate and dye carrier remain under vacuum clamping pressure. Again, the previously applied vacuum, 18, is maintained during this second thermal event that comprises the removal of thermal energy at 24. It has been found that a second, rapid cooling thermal event conducted under vacuum clamping pressure presents advantages over previous dye sublimation imaging technologies.

A first advantage accruing from this step is that the release of dye carrier 3 from substrate 1 is greatly improved over that of previous methodologies. Indeed, by carefully adjusting the temperature and duration of the first thermal event, in conjunction with the rapid cooling of the second thermal event, the unwanted adhesion of dye carrier 3 to substrate 1 has been completely obviated. While not wishing to be bound by theory, it is believed that the rapid cooling occasioned by the second thermal event introduces some thermal or mechanical shock between dye carrier 3 and substrate 1 which renders these two elements separable. This is accomplished without the need for special intermediate dye transfer webs or time-consuming pre-imaging conditioning processes required by other methodologies.

A second advantage afforded by this step relates to the previously discussed problems of unwanted distortion of the substrate caused by the heating and cooling thereof without benefit of a strong, evenly applied clamping force over the entire surface of the substrate. Again not wishing to be bound by theory, it is believed that prior dye sublimation technologies, by inducing the heating and cooling of the substrate, liberate internal forces within an unconstrained substrate which, on cooling, tend to twist, shrink, and warp the substrate. By utilizing an atmospheric clamping methodology including a tough, yet resilient membrane 16, for instance the vacuum clamping technology previously discussed, the present invention avoids this problem by forcing the retention of the substantially flat shape of the sheet throughout the several thermal events of the dye sublimation imaging process.

Referring now to FIGS. 1F-1H, at the completion of the second, cooling thermal event, vacuum is released at 26 and membrane 16 removed from the dye carrier—substrate stack at 28. Thereafter, dye carrier 3 and substrate 1 are lifted from passive cooling device 12. At this point the image carried by dye carrier 3 has been formed within substrate 1. The degree of dye penetration within the plastic is dependent upon several factors. These include sublimation temperature, clamping pressure, and duration of application of thermal energy and clamping pressure.

A first apparatus for performing a methodology according to the present invention is disclosed having reference to FIGS. 2A-2C and 3. Dye transfer apparatus 200 includes a table assembly 202, at least one platen assembly 204, a thermal imaging unit 206, and a vacuum system 240. Table assembly 202 supports thermal imaging unit 206 and platen assembly 204. As will be discussed later, the utilization of a plurality of platen assemblies 204 presents advantages with respect to manufacturing efficiency. Accordingly, in one preferred embodiment of this apparatus there are provided a pair of platen assemblies 204 and 204′, which are substantially identical. Table assembly 202 is preferably equipped so that platen assemblies 204 and 204′ may be introduced into thermal imaging unit 206 in rotation. In order to effect this insertion of platen assemblies 204 and 204′, a system of rollers or slides may be implemented. These rollers are preferably formed as a series of rollers 220 on table assembly 202, across which platen assembly 204 and 204′ slide. Alternatively, rollers or slides may be provided on the underside of platen assemblies 204 and 204′. Other sliding-friction reducing methods including air cushions, polished metal slides, and PTFE slides may, of course, be implemented.

Apparatus 200 is further equipped with a vacuum system 240. Vacuum system 240 includes a vacuum source, for instance a vacuum pump 242 and optionally a vacuum reservoir 244, which is connected by piping 245 to vacuum pump 242. This vacuum source is then connected by means of flexible piping 246 and vacuum valve 248 to platen assembly 204, and more particularly, to vacuum orifices 14 thereof. A similar set of piping and vacuum valves, 246′ and 248′ connects the vacuum source to platen assembly 204′. The actuation of vacuum valves 248 and 248′ may be manual, remote, or automated. In a preferred embodiment of this apparatus, vacuum valves 248 and 248′ are electrically controlled valves operated from a control station 300.

Referring now to FIG. 2B, details of thermal imaging unit 206 and platen assembly 204 are shown. Platen assembly 204 comprises a perforated aluminum platen 10, atop which is placed a passive cooling device 12. A frame, sometimes referred to herein as a “spectacle frame”, 216 is hingedly attached at one side to platen 10 by means of hinges 208. Frame 216 has mounted thereto a sheet of elastomeric membrane 16, previously discussed, which membrane covers the aperture formed by the frame. In this embodiment of the present invention membrane 16 takes the form of the textured sheet of DuPont Viton™.

Thermal imaging unit 206 in this embodiment is a substantially hollow box-like chassis 207 having mounted therein a heat source. This heat source may implement substantially any of the previously discussed heating methodologies, and in this embodiment of the present invention comprises at least one, and preferably a plurality of electrical infrared bulbs 208. Chassis 207 is hingedly attached to table assembly 202 by means of hinges 210 a suitable distance above the surface of table assembly 202 such that, when closed, thermal imaging unit 206 is positionable flushly atop platen assembly 204 when chassis 207 is lowered onto platen assembly 204.

The operation of apparatus 200 is further described having reference to FIGS. 2B-C. At the start of one imaging cycle, frame 216 is opened as shown at the left side of FIG. 2B, and a sheet of plastic substrate 1 is inserted atop cooling device 12. Thereafter, a dye carrier 3 having a dye image, 5, imprinted thereon is positioned such that dye image 5 is in direct contact with substrate 1. Dye image 5 may be advantageously formed by imprinting the image, utilizing substantially any known dye sublimation dyestuff, onto one surface of dye carrier 3. Thereafter, frame 216 is closed over the dye carrier-substrate-cooling device stack substantially as shown at the right side of FIG. 2B. Thereafter, vacuum valve 248 is opened, causing vacuum system 240 to evacuate the area under membrane 16. This evacuation seals membrane 16 to platen 10 and the previously discussed stack, and provides an atmospheric clamping force to effect the dye transfer process. It also acts to maintain the registration of the dye carrier 3 with respect to substrate 1, and in novel fashion to preclude the unwanted distortion of substrate 1, as previously discussed.

Once suitable vacuum clamping force has been obtained, platen assembly 204 is slidably positioned beneath chassis 207 of thermal imaging unit 206, and chassis 207 is positioned on top of platen assembly 204. It should be noted that the hinged elevation and lowering of both chassis 207 of thermal imaging unit 206, and frame 216 of platen assembly 204, may be manually performed, or advantageously may be performed by any lifting methodology known to those having skill in the art. These lifting methodologies include, but are specifically not limited to, pneumatic cylinders, hydraulic cylinders, servo motors, spring devices, counter weights, screw or geared devices and all other elevating and depression methodologies known to those having ordinary skill in the art.

After chassis 207 is lowered onto platen assembly 204, infrared bulbs 208 are energized causing the heating 22 of membrane 16, dye carrier 3 and substrate 1, while dye carrier 3 and substrate 1 remain under the previously discussed clamping vacuum. The temperature beneath membrane 16 may be monitored by means of a thermocouple 212 positioned between passive cooling device 12 and membrane 16. Alternative temperature monitoring methodologies known to those of ordinary skill in the art may, of course, be implemented. Infrared bulbs 208 remain energized for a specified time empirically determined to be optimal for the substrate, sublimation dyestuff, and degree of dye transfer imaging desired.

Once the specified thermal imaging time has elapsed, infrared bulbs 208 may be de-energized, chassis 207 is elevated, and platen assembly 204 is removed from under chassis 207 while a vacuum is retained under membrane 16. The advantages of an embodiment of this apparatus having two platen assemblies 204 and 204′ are now shown. While platen assembly 204 is cooled under vacuum clamping at 24, platen assembly 204′, having been previously loaded and vacuum clamped, is positioned under chassis 207 of thermal imaging unit 206. Chassis 207 is then lowered onto frame 216 of platen assembly 204′ and the heat source, for instance infrared bulbs 208, may again be energized. Accordingly, one of platen assemblies 204 and 204′ is cooling while the other is heating, while both retain their respective substrate/dye carrier stacks under clamping pressure.

In this embodiment of the present invention, which implements a passive cooling device, cooling 24 is accomplished by exposing platen assembly 204 to ambient air temperature. This exposure may be augmented by introducing a flow of ambient air across the surface of platen assembly 204, and most especially across membrane 16 by means of a fan or other airflow-inducing device. Passive cooling device 12 serves to passively cool substrate 1, dye carrier 3 and at least a portion of membrane 16 in the following manner: being of very low thermal mass, passive cooling device retains little unwanted heat. Once the first thermal event is complete, and the second thermal event commenced, passive cooling device 12 contributing little additional head, enables the rapid cooling of the membrane and the elements under it.

Again, the temperature beneath membrane 16 is monitored by means of thermocouple 212 until substrate 1 has reached a temperature sufficiently cool to return it to its rigid state without distortion. Thereafter, vacuum valve 248 is actuated to relieve the vacuum beneath membrane 16, thereby releasing the clamping pressure to substrate 1 and dye carrier 3. Once clamping pressure is released, substrate 1 and dye carrier 3 may be removed from platen assembly 204 after lifting frame 216 therefrom. By using two platen assemblies, for instance 204 and 204′, to alternatively heat and cool the substrate in this manner, imaging throughput is nearly doubled.

In one application of this embodiment of the present invention an 80 mil acrylic sheet was utilized as substrate 1. The acrylic sheet was imaged by positioning it on passive cooling device 12, and then superposing a dye carrier 3 thereover, as shown. Spectacle frame 216 was then lowered over the acrylic sheet and dye carrier 3, covering them with membrane 16. In this embodiment, a silicone rubber sheet was implemented as membrane 16. After evacuation of the space under membrane 16, processing proceeds in previously discussed for this embodiment. In this case the acrylic sheet was processed for 10 minutes at a temperature 350° F. After this first thermal event, cooling proceeded as previously discussed.

Where one embodiment of the present invention utilizes radiant heating, as previously discussed, another embodiment utilizes conductive heating. An example of such a conductive apparatus is shown having reference to FIGS. 4A-4C and 6. This embodiment shares many characteristics of the previously discussed apparatus, but with one significant difference: where the previously discussed apparatus used radiant heating, this embodiment utilizes a conductive heating plate, 402. Table 202 and vacuum system 240 are as before, as are platen assemblies 204 and 204′. In this embodiment however, thermal imaging unit 400 defines a lateral tunnel, 450, therethrough.

Conductive heating plate 402 is retractably mounted within a portion of thermal imaging unit 400 overlying tunnel 450. Conductive heating plate 402 in this embodiment is typically retracted upwardly by means of a retractor, in this embodiment one or more tension springs 406. Alternative retraction devices known to those having ordinary skill in the art, including but not limited to counterweights, pneumatic cylinders, vacuum cylinders, chains, bag presses, cables, hydraulic cylinders, and electromechanical devices may, with equal facility, be implemented.

Positioned between an upper surface, 403, of conductive heating plate 402 and an inner surface 401 of thermal imaging unit 400 is a clamping or urging device, which works in opposition to tension springs 406 to urge conductive heating plate 402 into contact with membrane 16. In the embodiment here illustrated, this urging device takes the form of a bag press, 404. Bag press 404 is connected to a controllable source of pressurized gas, or air by means of piping and valves, not shown in this figure, to effect the urging of heating plate 402 into contact with membrane 16. Of course, alternative clamping or urging devices known to those of ordinary skill in the art may also be implemented to fulfill this function. Non-limiting examples of such include hydraulic cylinders, pneumatic cylinders, magnetic urging devices to include electromagnetic urging devices, camshafts, crankshafts, wedges, and other methodologies for imparting substantially linear motion known to those of ordinary skill in the art.

Referring now to FIGS. 4B-4C and 10, which depict a cut-away section through thermal imaging unit 400, the operation of this embodiment is explained. At FIG. 4A, platen assembly 204 has just been removed from thermal imaging unit 400. Platen assembly 204′ has been loaded with substrate 1 and dye carrier 3, as before. At this point both platen assemblies 204 and 204′ are retained under vacuum clamping pressure by means of vacuum valves 248 and 248′ being opened to vacuum system 240. It will be appreciated that conductive heating plate 402 has been retracted upwardly by means of tension springs 406, rendering platen assembly 204′ slidably insertable into thermal imaging unit 400 by means of tunnel 450.

Conductive heating plate 402 may be heated by any means known to those having ordinary skill in the art including, but not limited to: electrical resistance wiring; the introduction of heated gases or fluids into a hollow interior portion 403 of conductive heating plate 402; radiant or convective heating of the conductive heating plate; open flame including one or more gas jets, as well as substantially any other methodology of controllably heating a conductive metal plate. In the embodiment depicted in the referenced figures, conductive heating plate 402 takes the form of a flat aluminum plate rendered partially hollow by the formation therein of at least one labyrinthine passage. In the embodiment depicted in this figure, conductive heating plate 402 is connected by means of piping and valves to an oil heater including an oil circulation pump. The oil heater thus provides a controllable flow of heated oil through the interior labyrinth of conductive heating plate 402 when it is desired to heat the plate. The flow of heated oil may be thermostatically controlled by hot oil valve 564 and hot oil piping 562 to retain conductive heating plate 402 at a desired temperature, or within a desired temperature range.

At FIG. 4C, platen assembly 204′ has been slidably inserted through tunnel 450 into thermal imaging unit 400, and conductive heating plate 402 urged into close contact with an upper surface of membrane 16′ by means of bag press 404. This urging is accomplished by introducing a controlled flow of compressed gas, or air, into an interior portion of bag press 404 by means of compressed air valve 450 and compressed air piping 452. Conductive heating plate 402, being heated to a temperature sufficient to induce the desired sublimation temperature in substrate 1 and dye carrier 3, effects that heating of the substrate through membrane 16 for the desired period of time. While this first thermal event is conducted in this fashion, platen assembly 204, having previously been removed from thermal imaging unit 400 is cooled as previously discussed, at 24.

Bag press 404 may also be used to increase the clamping pressure delivered to dye carrier 3 and substrate 1 above the nominal one atmosphere clamping pressure attainable by means of vacuum clamping alone. In one embodiment of the present invention, it is contemplated that the auxiliary clamping force attained by pressurizing the bag press may contribute as much as an additional 20 atmospheres of clamping pressure, where the substrate/sublimation dye combination warrants such elevated clamping pressures.

Once the temperature of substrate 1 has been elevated to the desired degree for the desired period of time, pressure within bag press 404 is relieved, again by means of compressed air valve 450 and compressed air piping 452. Conductive heating plate 402 is then retracted to the position shown at FIG. 4B by means of tension springs 406. Thereafter platen assembly 204′ is removed from thermal imaging unit 400 to the position shown at FIG. 4A, and platen assembly 204, having been re-loaded following the cooling of its previously imprinted substrate, is ready for insertion into thermal imaging unit 400.

In one application of this embodiment of the present invention an 80 mil Sintra® sheet was utilized as substrate 1. The Sintra® sheet was imaged by positioning it on passive cooling device 12, and then superposing a dye carrier 3 thereover, as shown. Spectacle frame 216 is then lowered over the Sintra® sheet and dye carrier 3, covering them with membrane 16. In this embodiment, a Viton® fluoroelastomer sheet was implemented as membrane 16. After evacuation of the space under membrane 16, processing proceeds as previously discussed for this embodiment. In this case the Sintra® sheet was processed for 5-7 minutes at a temperature of 285° F. Following this first thermal event, cooling proceeded as previously discussed.

While the embodiments of the present invention previously disclosed utilize a system of rollers, for instance 220, in order to render platen assemblies 204 slidably positionable under the thermal imaging head, the principles of the present invention contemplate alternative methodologies for positioning platen assemblies 204 and 204′ under thermal imaging unit 400. These methods include but are not limited to forcing one or more platen assemblies 204 vertically into position with respect to thermal imaging unit 400, and rotatably positioning such platen assemblies 204 below thermal imaging unit 400. Moreover, while platen assemblies 204 may be manually slid into position with respect to thermal imaging unit 400, the principles of the present invention further contemplate the use of a positioning member, not shown, to effect the slidable positioning of platen assembly 204. Examples of such positioning members include, but are again not limited to, pneumatic cylinders, hydraulic cylinders, gears, screw drives, gear drives, cables, chains, electrical coils, electromechanical devices, and other positioning methodologies well-known to those having ordinary skill in the art.

Referring now to FIGS. 5A-5B, 6, and 7, a further embodiment of the present invention implementing an active cooling system is disclosed. Again, table 202, thermal imaging unit 400, and vacuum system 240 are substantially as previously disclosed. This embodiment however introduces an active cooling element 550 in place of the previously discussed passive cooling device 12. According to one embodiment of the present invention, active cooling element 550 comprises a thermally conductive flat plate, for instance an aluminum plate, defining therein at least one internal cavity 551, which is connected to a cooling system 520. Cooling system 520 comprises piping 524 and valves 526 connecting active coolant element 550 to a cooling source 522.

Cooling source 522 may employ substantially any cooling or refrigeration methodology known to those having ordinary skill in the art. By way of illustration, but not limitation, such refrigeration methodologies include, but are not limited to the flow of refrigerated liquids and gases, the introduction into cavity 551 of a flow of super-cooled liquid, for instance liquid nitrogen, and the induction by means of a small orifice from cavity 551 of an expanding flow of gas, resulting in the cooling of element 551.

The implementation of the methodology taught herein is conducted utilizing this embodiment as follows: once platen assembly 204 has been introduced into thermal imaging unit 500 and aligned with respect to conductive heating element 502, a flow of compressed gas, for instance air, is introduced through valve 452 and pressure piping 450 into the interior of bag press 404, inflating bag press 404 and urging conductive heating element 502 downward into intimate contact with membrane 16. Conductive heating element 502 is retained in intimate contact with membrane 16 by maintaining pressure within bag press 404. This pressure may be maintained or regulated by means of a pressure regulator, not shown.

Conductive heating element 502 is heated, in this embodiment, by means of a controlled flow of heated oil from oil heater 560 introduced into an interior portion 503 of conductive heating element 502 through heating oil piping 562. The flow of heated oil is controlled by means of hot oil valve 564. An oil return line, not shown, returns cooled oil from conductive heating element 502 to oil heater 560. Thermocouple 212 measures the temperature under membrane 16.

Once the temperature under membrane 16 has reached the desired sublimation temperature, it is maintained at that temperature by means of a continuing flow of heated oil for the duration of the first, or heating, thermal event. Thereafter, pressure is released from bag press 404 by means of valve 452, and tension springs 406 retract conductive heating element 502 upwardly, compressing bag press 404. A controlled flow of chilled water is then introduced into an interior cavity 551 of active cooling plate 550 by means of chilled water piping 524 and chilled water valve 526 from cooling source 522 to effect the second, or rapid cooling, thermal event. A water return line, not shown, returns warmed water to cooling source 522 for re-cooling. Again, the temperature of substrate 1 is measured by thermocouple 212, and when substrate 1 has been returned to its substantially rigid state, the flow of chilled water into active cooling plate 550 is secured by means of chilled water valve 526. This concludes the second, or cooling, thermal event.

At this point platen assembly 204 is slidably retracted from thermal imaging unit 500 through tunnel 450 and the vacuum, which has been maintaining clamping pressure on substrate 1 and dye carrier 3, is released by means of vacuum valve 248 and vacuum piping 246. Thereafter frame 216 is lifted from platen 10 and both dye carrier 3 and substrate 1, now bearing the desired image, are removed from atop active cooling plate 550. Thereafter platen assembly 204′ may be introduced into thermal imaging unit 500 and processing repeated as described above. Platen assembly 204 may be advantageous re-loaded with another blank substrate 1 and dye carrier 3, and platen assembly 204 may then be readied for insertion into thermal imaging unit 500 following the previously discussed vacuum clamping process.

Yet another alternative is presented having reference to FIG. 8. This embodiment is similar to the embodiment depicted in FIG. 7, but with this difference: active cooling plate 550 is not utilized in the present environment, but both heating and active cooling are performed by means of a thermal plate 802. Thermal plate 802 is similar to conductive heating plate 502 shown in FIG. 7 with the exception that it provides both heating and active cooling to substrate 1 and dye carrier 3 through membrane 16. This is accomplished in the following manner: to effect the heating of thermal plate 802 there is introduced into an internal cavity 803 thereof a controlled flow of heated fluid, for instance a 50 percent mixture of ethylene glycol and water, this mixture sometimes hereafter referred to as “water”, by means of hot water piping 804 controlled by hot water valve 806.

This heated fluid may be heated in a furnace, boiler, or other fluid heating means known to those having ordinary skill in the art. Moreover, in order to attain substantially elevated temperatures without boiling this heated fluid, the principles of the present invention contemplate raising the pressure within the heating system to prevent the heated fluid from boiling. Processing proceeds as previously described until the desired duration of the first thermal event has been reached. At this point, the flow of hot water through hot water piping 804 is secured at hot water valve 806. Thereafter a flow of coolant, for instance chilled water, which in this embodiment will be understood to include the previously discussed mixture of water and ethylene glycol, is then introduced into the internal cavity, 803 of thermal plate 802 by means of chilled water piping 524 controlled by chilled water valve 526. This has the effect of rapidly cooling thermal plate 802 and transforming that unit into a cooling plate. Once again, cooling is applied to substrate 1 and dye carrier 3, now by chilled thermal plate 802, until such time as substrate 1 has been returned to its substantially rigid state. Thereafter, processing proceeds as previously described.

In one application of this embodiment of the present invention a 60 mil Kydex® sheet was utilized as substrate 1. The Kydex® sheet was imaged by positioning it on platen 10, and then superposing a dye carrier 3 thereover, as shown. Spectacle frame 216 is then lowered over the Kydex® sheet and dye carrier 3, covering them with membrane 16. In this embodiment, a butyl rubber-covered canvas sheet was implemented as membrane 16. After evacuation of the space under membrane 16, processing proceeds in previously discussed for this embodiment. In this case the Kydex® sheet was processed for 5 to 10 minutes at temperatures from 370° F. to 335° F. Following the first thermal event, a flow of chilled fluid was introduced into the interior, 803 of thermal plate 802 to cool the Kydex® sheet, substantially to room temperature.

An alternative to this embodiment contemplates the utilization of separate heating and cooling plates, either in the same thermal imaging unit, or in separate heating and cooling units. These units could be substantially identical, with the sole difference being the type of plate, either heating or cooling, employed therein.

Still another embodiment is shown having reference to FIG. 9. The apparatus shown in FIG. 9 is substantially identical with that described above and shown in FIG. 8, with the addition of passive cooling device 12. Accordingly, it will be appreciated that this embodiment employs both active and passive cooling. The utilization of a passive cooling device 12 in addition to thermal plate 802 has the advantage that any unwanted heat transferred by the first thermal event into platen 10 is minimized by the utilization of passive cooling device 12.

Another embodiment of the present invention is disclosed having reference to FIGS. 11A-F, 12, and 13. This embodiment utilizes a platen assembly 1000 similar to at least one of the previously described platen assemblies. Platen assembly 1000 is however designed to accommodate a plurality of substrate-dye sheet pairs arranged as a substrate stack, 1003. At FIG. 11A a substrate, 1 and dye carrier, 3 having image 5 imprinted thereon utilizing dye sublimation inks as previously disclosed, is loaded as before. Thereafter alternating layers of substrate 1 and dye carrier 3 are loaded until a substrate stack 1003 having the desired thickness is formed, a shown at FIG. 11B.

In this embodiment platen assembly 1000 comprises a platen 1001 substantially as previously discussed, but further incorporating some clamping device for holding frame 1010 to platen 1001, as shown at FIG. 11C. Frame 1010 is substantially like the previously disclosed frames, but has provisions for being clamped to platen 1001. In the embodiment presented in FIGS. 11A-F, 12 and 13, this clamping is effected by means of a plurality of bolts 1020 inserted through holes formed about the periphery of frame 1010 and thereafter inserted through a corresponding plurality of matching holes formed about the periphery of platen 1001. Thereafter a nut 1022 is threaded onto each of bolts 1020 to secure frame 1010 to platen 1001 and capturing, under membrane 16, substrate stack 1003. While the clamping together of frame 1010 and platen 1001 in this example has been effected by the simple expedient of utilizing a plurality of threaded nuts and bolts, study of the principles enumerated herein will elucidate to those having ordinary skill in the art that a wide variety of known clamping methodologies may, with equal facility may be implemented. These clamping methodologies include, but are specifically not limited to, patent clamps, over-center clamps, wedges, C-clamps and other threaded clamps, ratchet clamps, catches, magnetic catches, electromagnetic clamping devices, and the like. The principles of the present invention specifically contemplate all such known clamping alternatives.

After frame 1010 is clamped to platen 1001, as shown at FIG. 11D, a vacuum is applied at vacuum orifices 14, which vacuum forms the atmospheric clamping pressure previously disclosed at 20. Once atmospheric clamping 20 is applied, thermal energy is applied at 22 to raise the temperature of substrate stack 1003 to the desired sublimation temperature. This atmospheric clamping pressure is maintained on substrate stack 1003 for the balance of the imaging process until it is desirable to remove the several elements of substrate stack 1003 following processing. After the desired interval required for dye sublimation imaging, thermal energy is removed at 24 to enable the several sheets of substrate 1 to return to their substantially rigid state. Thereafter, nuts 1022 are removed from bolts 1020, bolts 1020 withdrawn from platen 1001 and frame 1010, and frame 1010 removed from platen 1001. This enables the removal from platen 1001 of the substrate stack 1003 and the separation of that stack into its component individual substrates 1 and dye carriers 3. At this point, as before, the image formed by reverse image 5 has been transferred into the several ones of the plurality of substrate 1 in substrate stack 103.

This embodiment of the present invention enables significantly longer imaging times. These lengthened imaging times present both advantages and challenges. One advantage is that a substrate may be imaged throughout the entire thickness of the substrate, resulting in a particularly rich, translucent image. The challenge in this case is to confine the image to one substrate. Long imaging times enable the unwanted migration of dyes from one substrate to another. This can lead not only to loss of resolution, but loss of registration accuracy, and uneven imaging throughout the several substrates of the substrate stack.

In order to preclude the unwanted migration between substrates, a dye stop may be inserted between individual substrate-dye carrier pairs. By placing such a dye stop, for instance a layer of metal foil, between a first substrate 1 and the dye carrier 3 of the substrate adjacent thereto, this unwanted dye migration may be obviated. The dye stop layer may, with equal facility, be implemented by forming the dye stop as a layer on the side of dye carrier 3 opposite to the image formed thereon, for instance by laminating a layer of metal foil.

While platen assembly 1000, disclosed in FIGS. 11A-F may be utilized in conjunction with any of the previously disclosed apparatuses, the utilization of this platen assembly enables large-scale batch processing of substrate material. One means for implementing such large-scale production utilizes at least one, and preferably a plurality of transportable platen racks 1200.

Platen rack 1200 in this embodiment takes the form of a movable shelf unit having at least one and preferably a plurality of shelves 1210 for receiving therein at least one and preferably a plurality of platen assemblies 1000. As it is desirable to maintain atmospheric clamping pressure by means of the vacuum applied to vacuum orifices 14 throughout the thermal events of the dye sublimation process taught herein, the several vacuum orifices 14 are connected by means of a vacuum hose 1204 to a vacuum reservoir 1220 for transportation from an area where platen assemblies 1000 may be loaded to an area where they may be imaged. Such an area is shown having reference to FIG. 13.

At FIG. 13 is shown an oven 1300 capable of receiving therein at least one, and preferably a plurality of platen racks 1200. Once the desired number of platen assemblies 1000 and platen racks 1200 has been received into an interior portion of oven 1300, oven door 1302 is secured, and the temperature within oven 1300 elevated to a temperature sufficient to achieve the thermal imaging temperature required by the several substrate elements of substrate stack 1003. This temperature may again be monitored by means of a thermocouple, inserted beneath membrane 16.

Once the desired imaging temperature has been reached and maintained for the desired imaging time, the several platen assemblies 1000 may be cooled to effect the second thermal event of the dye sublimation imaging process taught by the present invention. This cooling may be effected by the simple expedient of withdrawing platen racks 1200 from oven 1300 and allowing platen racks 1200 and platen assemblies 1000 to cool by natural air circulation. Alternatively, platen racks 1200, retaining platen assemblies 1000 therein, may be subjected to an active cooling process. Such active cooling may be effected by means of inserting platen racks 1200 into a refrigerator, or immersing them in a bath of chilled fluid. Alternatively, where oven 1300 is also equipped with a refrigeration capability, such refrigeration may be activated and oven 1300 may be utilized to implement an active cooling step.

In one application of this embodiment of the present invention an 80 mil Acrylonitrile-Butadiene-Styrene-Copolymer (ABS) sheet was utilized as substrate 1. A plurality of ABS sheets were imaged by superposing a dye carrier 3 and a dye stop layer 1003 atop each sheet. The plurality of ABS sheets were then positioned atop passive cooling device 12. Frame 1010 was then lowered, covering the several substrate-dye carrier-dye stop stacks with membrane 16. In this embodiment, a silicone rubber sheet was implemented as membrane 16. After evacuation of the space under membrane 16, processing proceeds in previously discussed for this embodiment. In this case the ABS sheets were processed for 3 hours at a temperature of 300° F. After this first thermal event, cooling proceeded as previously discussed.

It should be noted that the use of passive cooling device 12 is optional in this embodiment.

Yet another embodiment of the present invention is disclosed having reference to FIGS. 14A-E. This embodiment of the present invention implements a vacuum bagging approach to applying the previously discussed vacuum clamping. At FIG. 14A there is shown a vacuum bag or envelope, 600. Vacuum bag 600 may be advantageously formed from any number of heat-resistant, flexible materials which are substantially impervious to air transmission. By way of illustration, but not limitation, examples of such materials include silicone rubber sheeting, butyl rubber sheeting, heat-resistant plastics and other polymers, and impregnated fabrics. One material particularly suited for forming vacuum bag 600 of the present invention is a 2 mil advanced nylon sheeting available from GEM Polymer Corporation, PO Box 210 Lakeside Avenue, Delano, Pa. 18220-0210. This material is heat-resistant to temperatures in excess of 400 degrees Fahrenheit and is available either as custom-made bags, or as rolls of sheet film for user fabrication.

In order to evacuate the interior of vacuum bag 600, a vacuum probe, 602, is fitted therethrough. Vacuum probe 602 serves to form a vacuum-tight penetration through vacuum bag 600 and to attach a vacuum source, not shown, for evacuating the interior of vacuum bag 600. Any of several known vacuum probes may be utilized for this function: one such vacuum probe is a model VP36 available from Torr Technologies, Inc., 1435 22nd St. NW, Auburn, Wash. 98001. Alternative vacuum probes may of course be fitted.

An imaging stack, 640, is made up as follows: a substrate, 1, has superposed thereon a dye carrier 3, as previously discussed. Atop dye carrier 3 is placed a dye stop, 652, consisting of a sheet of aluminum foil. Alternatively, the side of dye carrier 3 opposite the image side thereof may have laminated thereon, or printed thereto, a layer of dye stop material, for instance a layer of metalized foil. A plurality of imaging stacks 640 are further stacked to form imaging body 650.

Imaging body 650 may optionally include one or more accessory layers, 654. Examples of these accessory layers include, but are not necessarily limited to, stiffening plates and pressure leveling layers. By way of illustration, but not limitation, stiffening plates may be formed of plywood, metal sheets, and composite materials, including honeycomb panels. Pressure levelers may be implemented utilizing pads of heat-resistant resilient foam, or other resilient materials. Again, in illustration but not limitation, one example of such heat-resistant foam is a layer of expanded silicone rubber foam.

Once imaging body 650 has been formed, it is inserted into opening 604 of vacuum bag 600 as shown at FIG. 14B. Thereafter, as shown at FIG. 14C, opening 604 is sealed at 606. Seal 606 may be effected by substantially any vacuum sealing methodology known to those having ordinary skill in the art. Again by way of illustration but not limitation, such seals include the thermal bonding of adjacent portions of opening 604, the use of sealing extrusions, cements, tapes, glues, clamps, and the closure of opening 604 followed by rolling at least a portion of opening 604 upon itself and thereafter securing that portion with clamps. Additionally, some of the previously discussed closure methods may implement the use of sealing compounds, putties or dopes to perfect the previously discussed vacuum seal.

Once seal 606 of vacuum bag 600 has been formed, vacuum bag 600 is evacuated at 620 through vacuum probe 602, forming evacuated imaging package 670. A vacuum source, not shown, is utilized to effect this evacuation. The evacuation of imaging package 670 may utilize vacuum piping, not shown, which is left in place during the imaging process, or may utilize a disconnect valve incorporating a vacuum check valve. This latter option enables imaging package 670 to be formed and removed from the vacuum source during imaging.

Following the formation of imaging package 670, it is heated to perform the previously discussed first thermal event. In one embodiment of the present invention, this heating is performed by means of inserting imaging package 670 into an oven, 700, thereafter closing oven doors 702, and heating the interior of oven 700 to a desired imaging temperature. Imaging package 670 is retained within oven 700 for a desired imaging period, and thereafter cooled, to perform the second thermal event. The cooling of imaging package 670 may be effected as previously discussed by means of withdrawing imaging package 670 from oven 700 and allowing the natural circulation of air to cool imaging package 670. Alternatively, imaging package 670 may be cooled by means of introducing a flow of cooled gas, fluid, or air about imaging package 670, or by immersing imaging package 670 into a body of cooled fluid.

Once imaging is completed, vacuum is released from within imaging package 670, seal 606 is opened, imaging body 650 removed therefrom, and the several elements thereof separated resulting in a plurality of imaged substrates, 1.

In one application of this embodiment of the present invention a plurality of 40 mil polycarbonate sheets, 30 inches by 60 inches, were imaged by forming imaging body 650 as discussed. Imaging package 670 was formed by evacuating vacuum bag 600 to a substantially complete vacuum. Thereafter, imaging package 670 was inserted into oven 700 and heated at 265° F. for a period of three hours. Following imaging, imaging package 670 was removed from oven 700 and allowed to cool utilizing natural air circulation. Once cooling was effected, vacuum was released and imaging body 650 separated into its component substrates, as discussed.

Reference was previously made to both those thermal events of a dye sublimation imaging cycle taught by the present invention, and to a control station for controlling the several events of such a dye sublimation imaging cycle. Each of these concepts is further explored having reference to FIG. 15. FIG. 15 is a graph of temperature over time of one dye sublimation imaging cycle utilizing active cooling.

FIG. 15 further includes a time line indicating the several control actions required to effect the dye sublimation imaging cycle. At time T₁ at least one platen is loaded with at least one substrate-dye carrier pair has previously discussed. At this point in time the temperature of the substrate-dye carrier pair is the ambient temperature. At time T₂ vacuum is applied to the several vacuum orifices 14 of platen assembly 204, effecting the previously discussed atmospheric clamping force. Thereafter, at T₃ platen assembly 204 is loaded into the thermal imaging unit of the apparatus. At time T₄ heat is applied through membrane 16 to dye carrier 3 and substrate 1. At time T₅ the required dye sublimation temperature has been attained. The time interval between T₄ and T₅ represents the time required to elevate the temperature of substrate 1 and dye carrier 3 to the required dye sublimation temperature. Accordingly, it will be appreciated that T₄-T₆ comprises the first thermal event taught by the present invention. At time T₆ the required dye sublimation time interval has been achieved, and the time between T₅ and T₆ represents this interval. At time T₆ heat is secured, and immediately thereafter at T₇ cooling is applied to substrate 1 and dye carrier 3. At time T₈ substrate three has been cooled to its substantially rigid temperature and cooling is secured. Accordingly, it will be appreciated that T₇-T₈ represent the second thermal event of the present invention. Thereafter at T₉ platen assembly 240 is removed from the thermal imaging unit. At time T₁₀ the vacuum is released beneath membrane 16, releasing the atmospheric clamping pressure of that membrane to substrate 1 and dye carrier 3. At T₁₁ the platen 10 may be unloaded and subsequently reloaded for another dye sublimation imaging cycle.

One or more of the control actions indicated may be performed manually, remotely, or automatically. An example of a manual control action is where a human operator manually operates a control element, for instance one of the previously disclosed vacuum, heating, or cooling valves. A remote control action is where a human operator utilizes a remote control, for instance a switch actuating a remotely controlled electrical valve, to initiate the control action. An automatic control action is where a sequencing device initiates a control action in response to a predetermined time interval or to a state indication. An example of such a state indication is where the temperature sensed by thermocouple 212 which could not only effect the ramp-up of temperature shown between T₄ and T₅, and the ramp-down of temperature shown between T₇ and T₈, but also could serve to alternately open and close a heating control valve to maintain the temperature specified between T₅ and T₆.

From the preceding discussion of imaging times, clamping pressures, imaging temperatures, and cooling times, it will be appreciated that the principles enumerated herein are applicable over a wide range of these variables. While the specifics of any given imaging regime are both highly specific and empirically determinable, in general terms, the present invention contemplates imaging temperatures for most plastic substrates at temperatures between 200° F.-600° F.; more particularly between 225° F. and 400° F., and more particularly still at temperatures between 250° F. and 300° F.

Similarly, imaging times of between 15 seconds and 12 hours have been shown to be advantageous for some embodiments of the present invention. More specifically, imaging times of between 1 minute and 1 may be implemented with advantage. Still more particularly, imaging times between 3 minutes and 15 minutes have been found satisfactory for some imaging regimes.

In like fashion, imaging pressures equating from 0.25 atmospheres to 20 atmospheres may be utilized to advantage. More particularly, such pressures from 0.5 to 5 atmospheres, and still more particularly, imaging pressures of 0.7 to 1.5 imaging pressures are satisfactory for a wide variety of plastic substrates.

From the foregoing discussion of several embodiments of the present invention, the ordering or spatial arrangement of the several elements of these embodiments was presented. It will be appreciated that these are by way of illustration and not limitation, and the present invention specifically contemplates modifications thereto.

Finally, while certain generic and patent plastic substrates have been presented as examples herein, the present invention has been found to be utile for imaging a vast array of different plastics. Accordingly, the principles of the present invention specifically contemplate the application thereof to a wide variety of plastics, and the examples presented herein are by way of illustration and not limitation.

The present invention has been particularly shown and described with respect to certain preferred embodiments of features thereof. However, it should be readily apparent to those of ordinary skill in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the invention as set forth in the appended claims. In particular, the principles of the present invention specifically contemplate the incorporation of one or more of the various features and advantages taught herein on a wide variety of dye sublimation apparatuses.

While examples of such alternatives have been presented herewith, it will be appreciated by those of skill in the art that alternative orientations of the several elements taught herein, alternative heating and cooling methodologies, clamping methodologies, and methods for positioning and indexing platen assemblies may, with equal facility be implemented to enable the features and advantages taught herein. Similarly, while discussion of the invention disclosed herein has centered on the utilization of that invention for forming dye sublimation images in solid sheets of plastic, study of the principles enumerated herein will elucidate to those having skill in the art that these principles are applicable to a wide variety of substrates including natural and man-made substances, films and polymer-coated materials, including polyesters. Each of these alternatives is specifically contemplated by the principles of the present invention. 

1. A method for forming a dye sublimation image in a thermoforming plastic substrate with a dye carrier having an image formed thereon of a sublimatic dyestuff, the method comprising: placing the image of the dye carrier against a first surface of the thermoforming plastic substrate to form a substrate-dye carrier stack; heating the substrate-dye carrier stack to a sublimation temperature; cooling the substrate-dye carrier stack; and providing a clamping pressure against the first surface of the thermoforming plastic substrate during the heating and the cooling and therebetween; wherein the clamping pressure is provided by: placing the thermoforming plastic substrate on a platen; covering the dye carrier, thermoforming plastic substrate, and at least a portion of the platen with a membrane; and applying a pressure differential across the membrane, wherein the clamping pressure limits warping of the substrate during the heating, cooling, and therebetween.
 2. The method, as recited in claim 1, wherein the clamping pressure provides a pressure over an entire surface of the dye carrier.
 3. The method, as recited in claim 1, wherein the clamping pressure provides a pressure of at least 14 pounds per square inch.
 4. The method, as recited in claim 1, wherein the cooling cools the substrate to a temperature which causes the substrate to be substantially rigid.
 5. The method, as recited in claim 1, wherein the heating heats to a temperature of between 200° F.-600° F.
 6. The method, as recited in claim 1, wherein the clamping pressure provides a pressure of between 0.25 atmospheres to 20 atmospheres.
 7. The method, as recited in claim 1, wherein the clamping pressure is provided by a gas pressure differential to provide the continuous pressure.
 8. The method, as recited, in claim 1, wherein the heating is to a temperature and for a period of time requisite to sublimate the image into the substrate.
 9. The method, as recited in claim 1, wherein the platen is a perforated platen with perforations forming a plurality of vacuum orifices, wherein the applying a differential pressure comprises applying a vacuum through the plurality of vacuum orifices. 